High pressure sodium vapor lamps are well known in the art and are widely used for street, roadway and area lighting applications. The lamps comprise an alumina ceramic tube which contains a charge of sodium or sodium plus mercury and is generally enclosed within an outer glass envelope or jacket. The lamps are conventionally operated on 60 cycle alternating current power by means of ballasts designed to limit the current and provide a power input not exceeding the lamp wattage rating.
The light generated by the discharge through the sodium or sodium plus mercury vapor is due almost exclusively to the excitation of the sodium atom through the self-reversal and broadening of the sodium D line at 589 nanometers. In those lamps containing mercury, the mercury serves as a buffer gas which raises the voltage gradient and thereby the efficacy but it is not excited to appreciable emission. The result is a lamp which is extremely efficient in terms of lumens per watt, for instance from 75 to 130 lumens per watt depending upon lamp size, efficacy increasing with size from 70 watts to 1000 watts. But the lamp is low in color temperature, from 2000.degree. to 2100.degree. Kelvin, and low in color rendition index, from 10 to 20. While object colors in all portions of the spectrum are recognizable, those at the "cool" end such as violets, blues, and to some extent greens are muted or grayed down. The lamp has proved suitable for most outdoor applications but is not generally acceptable for indoor applications, particularly where critical color discrimination is required.
It was recognized in U.S. Pat. No. 3,248,590 -- Schmidt, that improved color rendition with the high pressure sodium vapor lamp could be achieved by going to higher sodium vapor pressure but at the price of a drop in efficacy. One line of attempts at improving the lamp's color temperature and rendition has been along the course suggested by Schmidt, namely raising the sodium vapor pressure, by one means or another. For instance U.S. Pat. No. 3,716,743 -- Mizuno et al, proposes to do so by heat shields about the lamp ends. Raising the sodium vapor pressure is similar to overwattaging the lamp, that is, operating it above its design rating; by so doing the color temperature may be raised but at the cost of a loss of about 10 lumens per watt in efficacy for each 100.degree. K. gain in color temperature 2100.degree. K. Also overwattaging can greatly accelerate sodium loss which leads to short term voltage rise and outer jacket darkening, and thus short life.
Other attempts at improving color temperature and rendition involved the addition of other elements to the lamp fill. For instance U.S. Pat. No. 3,521,108 -- Hanneman, proposed the addition of cadmium and optionally thallium to the sodium and mercury. None of these attempts up to the present has resulted in a lamp or lighting system which is a practical commercial product because the improvements were minor or outweighed by the concomitant disadvantages.